Biodiesel, a renewable fuel with similar combustion properties to fossil diesel, is normally produced by transesterification of highly refined oils with short-chain alcohols. Biodiesel can significantly decrease the exhaust emission of CO2, SQ and unburned hydrocarbons from motor vehicles. Biodiesel is environmentally beneficial, and therefore, is a promising alternative to fossil diesel.
Conventionally, biodiesel is produced by a transesterification process using homogeneous strong base catalysts, such as sodium hydroxide or potassium hydroxide. The strong base catalysts have many advantages such as high activity and mild reaction conditions. However, these homogeneous base catalysts are corrosive, and removal of the catalysts from the biodiesel product is not straight forward. In general, a large amount of waste washing water is generated, and a long time is required for phase separation. The use of solid base catalysts can alleviate these problems and thus, recently highly active solid base catalysts have been developed for biodiesel production.
Some calcium-containing solid base catalysts were reported to be active in oil transesterification with methanol. Several traditional methods have been described for the preparation of the calcium-containing catalysts, such as CaO catalysts supported on mesoporous silica using an impregnation method, solid super bases prepared by dipping CaO in an ammonium carbonate solution, and CaTiO3, CaMnO3, Ca2Fe2O5, CaZrO3, and Ca—CeO2 catalysts prepared by a physical mixing method. Solid base catalysts were also prepared using a precipitation method.